NOTE: this format is still experimental and
subject to change without backwards compatibility.

Basic idea:

Packed Blocks and VInt Blocks:

In packed blocks, integers are encoded with the same bit width (packed format):
the block size (i.e. number of integers inside block) is fixed (currently 128). Additionally blocks
that are all the same value are encoded in an optimized way.

In VInt blocks, integers are encoded as VInt:
the block size is variable.

Block structure:

When the postings are long enough, Lucene41PostingsFormat will try to encode most integer data
as a packed block.

Take a term with 259 documents as an example, the first 256 document ids are encoded as two packed
blocks, while the remaining 3 are encoded as one VInt block.

Different kinds of data are always encoded separately into different packed blocks, but may
possibly be interleaved into the same VInt block.

The structure of skip table is quite similar to previous version of Lucene. Skip interval is the
same as block size, and each skip entry points to the beginning of each block. However, for
the first block, skip data is omitted.

Positions, Payloads, and Offsets:

A position is an integer indicating where the term occurs within one document.
A payload is a blob of metadata associated with current position.
An offset is a pair of integers indicating the tokenized start/end offsets for given term
in current position: it is essentially a specialized payload.

When payloads and offsets are not omitted, numPositions==numPayloads==numOffsets (assuming a
null payload contributes one count). As mentioned in block structure, it is possible to encode
these three either combined or separately.

In all cases, payloads and offsets are stored together. When encoded as a packed block,
position data is separated out as .pos, while payloads and offsets are encoded in .pay (payload
metadata will also be stored directly in .pay). When encoded as VInt blocks, all these three are
stored interleaved into the .pos (so is payload metadata).

With this strategy, the majority of payload and offset data will be outside .pos file.
So for queries that require only position data, running on a full index with payloads and offsets,
this reduces disk pre-fetches.

The .tim file contains the list of terms in each
field along with per-term statistics (such as docfreq)
and pointers to the frequencies, positions, payload and
skip data in the .doc, .pos, and .pay files.
See BlockTreeTermsWriter for more details on the format.

NOTE: The term dictionary can plug into different postings implementations:
the postings writer/reader are actually responsible for encoding
and decoding the Postings Metadata and Term Metadata sections described here:

Header is a CodecHeader storing the version information
for the postings.

PackedBlockSize is the fixed block size for packed blocks. In packed block, bit width is
determined by the largest integer. Smaller block size result in smaller variance among width
of integers hence smaller indexes. Larger block size result in more efficient bulk i/o hence
better acceleration. This value should always be a multiple of 64, currently fixed as 128 as
a tradeoff. It is also the skip interval used to accelerate DocIdSetIterator.advance(int).

DocFPDelta determines the position of this term's TermFreqs within the .doc file.
In particular, it is the difference of file offset between this term's
data and previous term's data (or zero, for the first term in the block).On disk it is
stored as the difference from previous value in sequence.

PosFPDelta determines the position of this term's TermPositions within the .pos file.
While PayFPDelta determines the position of this term's <TermPayloads, TermOffsets?> within
the .pay file. Similar to DocFPDelta, it is the difference between two file positions (or
neglected, for fields that omit payloads and offsets).

PosVIntBlockFPDelta determines the position of this term's last TermPosition in last pos packed
block within the .pos file. It is synonym for PayVIntBlockFPDelta or OffsetVIntBlockFPDelta.
This is actually used to indicate whether it is necessary to load following
payloads and offsets from .pos instead of .pay. Every time a new block of positions are to be
loaded, the PostingsReader will use this value to check whether current block is packed format
or VInt. When packed format, payloads and offsets are fetched from .pay, otherwise from .pos.
(this value is neglected when total number of positions i.e. totalTermFreq is less or equal
to PackedBlockSize).

SkipFPDelta determines the position of this term's SkipData within the .doc
file. In particular, it is the length of the TermFreq data.
SkipDelta is only stored if DocFreq is not smaller than SkipMinimum
(i.e. 8 in Lucene41PostingsFormat).

SingletonDocID is an optimization when a term only appears in one document. In this case, instead
of writing a file pointer to the .doc file (DocFPDelta), and then a VIntBlock at that location, the
single document ID is written to the term dictionary.

Term Index

The .tip file contains an index into the term dictionary, so that it can be
accessed randomly. See BlockTreeTermsWriter for more details on the format.

Frequencies and Skip Data

The .doc file contains the lists of documents which contain each term, along
with the frequency of the term in that document (except when frequencies are
omitted: FieldInfo.IndexOptions.DOCS_ONLY). It also saves skip data to the beginning of
each packed or VInt block, when the length of document list is larger than packed block size.

Calculate the difference between each document number and previous one,
and get a d-gaps list (for the first document, use absolute value);

For those d-gaps from first one to PackedDocBlockNum*PackedBlockSizeth,
separately encode as packed blocks.

If frequencies are not omitted, PackedFreqBlock will be generated without d-gap step.

VIntBlock stores remaining d-gaps (along with frequencies when possible) with a format
that encodes DocDelta and Freq:

DocDelta: if frequencies are indexed, this determines both the document
number and the frequency. In particular, DocDelta/2 is the difference between
this document number and the previous document number (or zero when this is the
first document in a TermFreqs). When DocDelta is odd, the frequency is one.
When DocDelta is even, the frequency is read as another VInt. If frequencies
are omitted, DocDelta contains the gap (not multiplied by 2) between document
numbers and no frequency information is stored.

For example, the TermFreqs for a term which occurs once in document seven
and three times in document eleven, with frequencies indexed, would be the
following sequence of VInts:

PackedDocBlockNum is the number of packed blocks for current term's docids or frequencies.
In particular, PackedDocBlockNum = floor(DocFreq/PackedBlockSize)

TrimmedDocFreq = DocFreq % PackedBlockSize == 0 ? DocFreq - 1 : DocFreq.
We use this trick since the definition of skip entry is a little different from base interface.
In MultiLevelSkipListWriter, skip data is assumed to be saved for
skipIntervalth, 2*skipIntervalth ... posting in the list. However,
in Lucene41PostingsFormat, the skip data is saved for skipInterval+1th,
2*skipInterval+1th ... posting (skipInterval==PackedBlockSize in this case).
When DocFreq is multiple of PackedBlockSize, MultiLevelSkipListWriter will expect one
more skip data than Lucene41SkipWriter.

SkipDatum is the metadata of one skip entry.
For the first block (no matter packed or VInt), it is omitted.

DocSkip records the document number of every PackedBlockSizeth document number in
the postings (i.e. last document number in each packed block). On disk it is stored as the
difference from previous value in the sequence.

DocFPSkip records the file offsets of each block (excluding )posting at
PackedBlockSize+1th, 2*PackedBlockSize+1th ... , in DocFile.
The file offsets are relative to the start of current term's TermFreqs.
On disk it is also stored as the difference from previous SkipDatum in the sequence.

Since positions and payloads are also block encoded, the skip should skip to related block first,
then fetch the values according to in-block offset. PosFPSkip and PayFPSkip record the file
offsets of related block in .pos and .pay, respectively. While PosBlockOffset indicates
which value to fetch inside the related block (PayBlockOffset is unnecessary since it is always
equal to PosBlockOffset). Same as DocFPSkip, the file offsets are relative to the start of
current term's TermFreqs, and stored as a difference sequence.

PayByteUpto indicates the start offset of the current payload. It is equivalent to
the sum of the payload lengths in the current block up to PosBlockOffset

Positions

The .pos file contains the lists of positions that each term occurs at within documents. It also
sometimes stores part of payloads and offsets for speedup.

PositionDelta is, if payloads are disabled for the term's field, the
difference between the position of the current occurrence in the document and
the previous occurrence (or zero, if this is the first occurrence in this
document). If payloads are enabled for the term's field, then PositionDelta/2
is the difference between the current and the previous position. If payloads
are enabled and PositionDelta is odd, then PayloadLength is stored, indicating
the length of the payload at the current term position.

For example, the TermPositions for a term which occurs as the fourth term in
one document, and as the fifth and ninth term in a subsequent document, would
be the following sequence of VInts (payloads disabled):

4, 5, 4

PayloadData is metadata associated with the current term position. If
PayloadLength is stored at the current position, then it indicates the length
of this payload. If PayloadLength is not stored, then this payload has the same
length as the payload at the previous position.

OffsetDelta/2 is the difference between this position's startOffset from the
previous occurrence (or zero, if this is the first occurrence in this document).
If OffsetDelta is odd, then the length (endOffset-startOffset) differs from the
previous occurrence and an OffsetLength follows. Offset data is only written for
FieldInfo.IndexOptions.DOCS_AND_FREQS_AND_POSITIONS_AND_OFFSETS.

Payloads and Offsets

The .pay file will store payloads and offsets associated with certain term-document positions.
Some payloads and offsets will be separated out into .pos file, for performance reasons.

The order of TermPayloads/TermOffsets will be the same as TermPositions, note that part of
payload/offsets are stored in .pos.

The procedure how PackedPayLengthBlock and PackedOffsetLengthBlock are generated is the
same as PackedFreqBlock in chapter Frequencies and Skip Data.
While PackedStartDeltaBlock follows a same procedure as PackedDocDeltaBlock.

PackedPayBlockNum is always equal to PackedPosBlockNum, for the same term. It is also synonym
for PackedOffsetBlockNum.

SumPayLength is the total length of payloads written within one block, should be the sum
of PayLengths in one packed block.

PayLength in PackedPayLengthBlock is the length of each payload associated with the current
position.

WARNING: This API is experimental and might change in incompatible ways in the next release.

fieldsProducer

Reads a segment. NOTE: by the time this call
returns, it must hold open any files it will need to
use; else, those files may be deleted.
Additionally, required files may be deleted during the execution of
this call before there is a chance to open them. Under these
circumstances an IOException should be thrown by the implementation.
IOExceptions are expected and will automatically cause a retry of the
segment opening logic with the newly revised segments.